Conventional swept frequency sources, or sweep oscillators, often lack the frequency stability and accuracy associated with high stability, fixed frequency oscillators. When swept frequency sources are employed in spectrum analyzers, these frequency imprecisions can lead to visible drift and frequency position errors of the displayed spectrum. This drift, in turn, can lead to inaccurate data collection or may delay collection of accurate data while the oscillator frequency is recalibrated to enable an accurate spectral measurement.
Phase-locked loops are often used as programmable frequency sources where both programmability and stability are required. Phase-locked loops operate by including a voltage-controlled oscillator (VCO), a programmable frequency divider, a phase detector, and a loop filter, connected to form a loop.
Also required is an external fixed-frequency oscillator, connected to the phase detector such that a signal derived from the VCO is phase-locked to the external fixed-frequency oscillator, as is well known in the art. This type of arrangement provides, via the phase-locking mechanism, the stability of the fixed-frequency oscillator while allowing VCO frequency programmability via alteration of the signal derivation process. However, such circuits require a certain amount of time, referred to as the "settling time", in order to accommodate VCO frequency changes. This arises because overall "loop" implementing a small frequency step typically has a fairly narrow bandwidth and so has a relatively long impulse response. When a change is introduced into the loop, for example to change the VCO frequency, the loop impulse response time determines the overall time required for the phase-locked loop to settle to the altered frequency.
What is needed is a simple means for providing a swept frequency source having stability characteristics similar to those associated with phase-locked loop frequency sources but having rapid settling times.
Accordingly, one advantage of the present invention is to provide a stable sweep oscillator incorporating the cost and performance advantages of phase-locked loops and having sweep rates which are independent of the phase-locked loop bandwidth.